Disk brake rotors for automotive applications have in the past and still are fabricated of cast iron utilizing conventional foundry casting techniques in which molten iron is gravity cast into a mold cavity formed between separable sand mold halves through a conventional gating system encircling the cavity having one or more ingates extending into the cavity for introducing the molten metal therein.
More recently, however, automotive manufacturers are looking to alternatives to conventional cast iron brake rotors and have found aluminum metal matrix composite material to be a suitable alternative, providing increased performance and wear and a significant reduction in weight as compared to their cast iron counterparts. Such aluminum metal matrix material, however, is very costly in comparison and more difficult to cast. Consequently, the gating arrangement conventionally employed for casting iron brake rotors may not be used to cast aluminum metal matrix composite rotors, since aluminum is poured at a lower temperature and has a tendency to cool to an unacceptably low pouring temperature in the gating system as well as picking up unacceptably high levels of hydrogen and other impurities before entering the cavity. Even if such a gating system could be used, however, the scrap metal material remaining in the gating system would render the usage of such aluminum matrix composite material cost prohibitive.
Cast aluminum composite brake rotors thus far have been produced by direct pouring the molten composite material into the cavity through a down sprue. No gating system is used. The temperature of the composite material is maintained and hydrogen pickup minimized. A filter is usually placed in the down sprue to trap oxides, slag, and other impurities from entering into the mold. Typical of such known direct pour systems is disclosed in U.S. Pat No. 4,928,746 to Butler et al, granted May 29, 1990. A ceramic foam filter is fixed inside a sleeve of refractory material that lines the down sprue of the mold. Such a sleeve/filter arrangement has been used in the past to produce brake rotors. The sleeve and filter joined to the mold by either inserting the sleeve into a down sprue of a prefabricated upper cope section, or else the sleeve is joined in situ with the making of the cope section. The cope section is joined to a drag section along a horizontal parting plane. The cope section has a central hub-forming portion projecting into the drag section to produce a corresponding central hub portion of the brake rotor. The sprue extends through the central hub portion into the mold cavity. A ring-shaped core is printed into the drag section and encircles the hub-forming portion for producing a ventilated disk portion of the rotor.
During formation of the cope section in which mold sand is rammed into a mold box around the refractory sprue sleeve, the sleeve may flex radially inward when subjected to the ramming forces causing a gap to be formed between the sleeve and the sprue wall. Such a gap is undesirable as it allows metal to flow around the sleeve thereby bypassing the filter to the detriment of casting quality. Such sleeves also have a tendency to shift downward during or after formation of the cope section under their own weight or when handled causing the lower end to extend beyond the cope section further into the mold cavity than designed, thereby narrowing the gap clearance between the exit of the down sprue and the underlying mold cavity wall of the drag section. It is important that this gap be carefully controlled since it governs the flow rate of molten metal into the mold cavity.